Minoxidil is a potent, long-acting peripheral vasodilator used for treatment in patients with severe hypertension. Although minoxidil has a low order of toxicity, it has been found to cause several types of pathologic alterations in the heart consisting of left ventricular and right atrial fibrosis. The etiology of these pathologic alterations is believed to be due to hemodynamic changes caused by minoxidil. We measured the influence of minoxidil on regional myocardial blood flow and attempted to correlate blood changes with histologic changes in these same regions. We found that minoxidil significantly increased myocardial blood flow to all regions of the heart except to the anterior and posterior papillary muscles of the left ventricle. Minoxidil significantly decreased mean aortic pressure, but significantly increased heart rate and cardiac output. Interestingly, LVEDP, pulmonary artery wedge and right sided pressures were significantly increased with minoxidil treatment. Although the histologic examinations are still being performed, it can be speculated that the previously seen myocardial necrosis of the papillary muscles may be on an ischemic basis with an increased work and end-diastolic pressure of the left ventricular but constant myocardial blood flow.